In optical transmission systems, especially in optical transmission systems in which a signal light is amplification-repeated and/or branched off in the middle, it is necessary to remotely control optical repeaters or branching units. Also, in optical networks, it is necessary to transmit network control information separately from the signal light.
Therefore, an arrangement has been proposed in which the signal light whose signal is conventionally transmitted as NRZ pulses, RZ pulses or soliton waves is again intensity-fluctuated with a control signal of relatively low frequency and then propagated on optical fiber transmission lines.
As means for superimposing or multiplexing the control light in the signal light, an arrangement has been well known that uses a structure of optical amplifier consisting of an erbium doped optical fiber, an intensity of its pumping light being modulated with the control signal. That is, gain of an erbium doped optical fiber is fluctuated in accordance with the control signal and, thus, intensity level of the signal light is fluctuated according to the control signal.
However, the gain fluctuation of the erbium doped optical fiber can follow approximately 100 kHz at the most. This is not enough for superimposing the control signal.
Furthermore, in the conventional systems, when the pumping light is disappeared or the intensity is largely reduced due to trouble of a pumping light source or the like, the erbium doped optical fiber plays a role of loss medium and considerably attenuates the signal light.
An object of the invention is to solve the foregoing problems and provide a control signal superimposer for superposing the control signal on the signal light.
Namely, the object of the invention is to provide a control signal superimposer capable of responding at high speed.
An another object of the invention is to provide a control signal superimposer capable of transmitting the signal light with substantially no loss even in case of trouble of the pumping light.
A further object of the invention is to provide a control signal superimposer not causing any harmful influence such as nonlinear effect to the signal light itself.